The hydrocarbon potential of the southern rift basins in the Gulf of Aden has been overlooked in the past, as indicated by vintage seismic. TGS’ onshore multi-client aeromagnetics in Somaliland (34,693 km) showcases several structural domains that correlate with rift related oil fields on the conjugate margin of onshore and offshore Yemen.

Somaliland lies at the southern margin of the Gulf of Aden, a young and narrow oceanic basin formed in Oligocene-Miocene time. The basin resides between two rifted margins of the Arabian and African Plates. The conjugate northern margin of Yemen has producing oil and gas fields and suggests the Somalian counterpart holds promising potential that is not yet completely explored.

The geology of Somaliland margin is similar to the Yemen margin until the Late Oligocene/Early Miocene. The similarities in fault trends and structural complexity of the Balhaf Graben/Mukalla High and the Berbera Basin/Erigavo High are striking and encouraging for the hydrocarbon prospectivity story.

Hydrocarbon exploration activity in Somaliland has been limited, with two offshore wells, both of which with oil and gas shows. Oil seeps onshore prove the existence of a petroleum system. A number of onshore wells in the Dagah Shabel area, in the Nogal Graben, and the latest well drilled onshore just across the border in Somalia in 2012 all encountered oil and gas shows in Mesozoic reservoirs.

Figure 1: TGS horizontal gradient of offshore shipborne magnetics and vertical gradient of onshore airborne magnetics with structural interpretation of the potential field maps in dark blue and well location in black.

When we compare the geological structures interpreted from the marine magnetics horizontal gradient, to the structural domains identified onshore from the airborne magnetics vertical gradient, the main regimes can be correlated. This enables the extrapolation of the geological domains identified offshore on seismic data, to the onshore, where only sparse vintage and no modern seismic data is publicly available.

The 2007 TGS airborne magnetic survey acquired 34,693 km of data onshore in conjunction with 5,398 km of 2D seismic data shot offshore. The airborne data onshore was acquired in two different phases, which resulted in a 5 km x 25 km initial grid covering the entire area and a 2.5 km x 2.5 km grid with the infill lines covering the coastal stretch and the main area in the south.

The magnetic line data onshore contains a wide spectrum of frequencies and permits detailed mapping, especially in the denser area of infill.

The 2.5 km line spacing of the regional grid is already of good resolution, where small scale structures of under 1.5 km width and 0.5 km depth are captured. The NW tip of the survey features densely clustered small-scale features, most likely at a shallow depth which may be related to the Miocene Volcanism onshore further to the NW (Djibouti).

The 25 km magnetic grid spacing ensures that basement depths of under 5 km are well resolved, and a minimum survey width of ~100 km permits the resolution of structures down to ~17 km. This includes the rift related basin structures identified in the coastal area to the east. The basin features are trending WNW-ESE and are perpendicular to the curved branches of the onshore strike-slip faults, which continue as transform faults in the Gulf of Aden. This implies that the rift basins and related sedimentary traps identified on offshore seismic are later stage rift structures subsequent to the rift segments located onshore.